1. Field of the Invention
The present invention relates to a fuel cell, and in particular, to a fuel cell capable of maintaining high power generation performance even when the external environment changes, and enabling miniaturization of a fuel cell system.
2. Description of the Background Art
In recent years, fuel cells as compact power sources for portable electronic devices that support an information society have been much expected because of their potential ability of offering high electrical efficiency as a single power supply. A fuel cell is a chemical cell that supplies a portable electronic device or the like with electrons by utilizing such an electrochemical reaction that a fuel such as hydrogen gas or a methanol aqueous solution is oxidized at an anode, and oxygen in air is reduced at a cathode.
Among a variety of fuel cells, a polymer electrolyte membrane fuel cell (hereinafter, referred to as “PEMFC”) using a proton-exchanged ion exchange membrane as an electrolyte membrane has the potential to practical application as a compact power source, because it provides high electrical efficiency in an operation at such a low temperature as 100° C. or less, and eliminates the necessity of heat application from outside in contrast to the fuel cell that operates at high a temperature such as a phosphoric-acid fuel cell or a solid oxide fuel cell, and thus requires no massive auxiliaries.
As a fuel that is supplied to such a PEMFC, for example, hydrogen gas using a hyperbaric gas cylinder, or mixture gas of hydrogen gas and carbon dioxide gas obtained by decomposing an organic liquid fuel by a reformer is used.
Since a direct methanol fuel cell (hereinafter referred to as “DMFC”) which generates electricity by supplying an anode of a PEMFC with a methanol aqueous solution and directly drawing out protons and electrons from the methanol aqueous solution needs no reformer, it has higher potential to practical use as a compact power source compared to a PEMFC. Furthermore, since it uses a methanol aqueous solution which is liquid under atmospheric pressure as a fuel, it can handle a fuel having a high volume energy density in a simple container without using a high pressure gas cylinder. Therefore, it realizes excellent safety as a compact power source and enables downsizing of the fuel container. Therefore, the DMFC attracts attentions from the viewpoint of application to a compact power source of a portable electronic device, in particular, application as an alternative secondary cell for a portable electronic device.
In a DMFC, the following reactions occur at an anode and a cathode.Anode: CH3OH+H2OCO2+6H++6e−Cathode: O2+4H++4e−2H2O
Theoretically, since methanol and water react at a molar ratio of 1:1, it is necessary to supply the anode with water in addition to methanol. When a liquid fuel having a high fuel concentration is used, a methanol aqueous solution of such a low concentration as 3 to 10% by mass is generally used because reduction in power generation performance due to crossover of the fuel in the solid polymer electrolyte membrane used as an electrolyte is large.
Methanol and water may be stored in advance in a fuel tank as a mixture of methanol and water, however, by collecting water generated at the cathode and supplying the anode with the water, the tank can be miniaturized because methanol can be stored in the fuel tank at a high concentration.
Generally, the water generated at the cathode is collected by auxiliaries that require an electric power such as a pump, and the generated water thus collected is supplied to the anode. However, use of auxiliaries such as a pump will limit miniaturization of the fuel cell. Therefore, it is effective for miniaturization of a fuel cell system to transport water to the anode by utilizing a water concentration gradient between the cathode and the anode in an electrolyte membrane rather than using auxiliaries such as a pump.
In order to achieve stable transportation of water to the anode by utilizing the water concentration gradient, it is necessary to keep the water concentration on the cathode side in the electrolyte membrane high, and thus to keep the humidity and the water concentration in the atmosphere of the cathode catalyst layer high.
However, in the case where gas permeability of the cathode diffusion layer is made small for the purpose of suppressing release of water, the water amount in the cathode diffusion layer is excess when the humidity of air of the external environment supplied to the cathode increases, so that the output is decreased due to flooding and power generation performance of the fuel cell is deteriorated.
On the other hand, in the case where gas permeability of the cathode diffusion layer is ensured in consideration of increase in the humidity of air of the external environment supplied to the cathode, the relative humidity of the atmosphere of the cathode catalyst layer decreases when the operation temperature increases or when the humidity of air of the external environment decreases, which promotes evaporation of water. This makes it difficult to keep an appropriate water concentration gradient in the electrolyte membrane.
In consideration of such a circumstance, for example, Japanese Patent Laying-Open No. 2006-172960 discloses a fuel cell capable of varying water drainability in the diffusion layer depending on the water content in the fuel cell without using control by various sensors and the like. Concretely, the fuel cell disclosed in Japanese Patent Laying-Open No. 2006-172960 has such a structure that a diffusion layer and a catalyst layer are sequentially stacked on each face of the electrolyte membrane, and two separators are disposed to sandwich the electrolyte membrane, and a water-absorbable resin that increases/decreases the volume depending on the water absorption amount is provided between two separators.
To be more specific, when the water amount in the fuel cell increases, the water-absorbable resin absorbs water to increase its volume, so that the power of the separators pushing the electrolyte membrane is reduced, and the thickness of the diffusion layer is increased. As a result, water easily travels in the diffusion layer, and water drainability in the diffusion layer increases.
On the other hand, when the water amount in the fuel cell is reduced, the volume of water-absorbable resin decreases due to water evaporation, and the power of the separators pushing the electrolyte membrane increases so that the thickness of the diffusion layer is reduced. As a result, water travels in the diffusion layer with difficulty, and drainability of water in the diffusion layer is reduced.